Stents, grafts, stent-grafts, vena cava filters and similar implantable medical devices, collectively referred to hereinafter as stents, are radially expandable endoprostheses which are typically intravascular implants capable of being implanted transluminally and enlarged radially after being introduced percutaneously. Stents may be implanted in a variety of body lumens or vessels such as within the vascular system, urinary tracts, bile ducts, etc. Stents may be used to reinforce body vessels and to prevent restenosis following angioplasty in the vascular system. They may be self-expanding, mechanically expandable or hybrid expandable.
Stents are generally tubular devices for insertion into body lumens. However, it should be noted that stents may be provided in a wide variety of sizes and shapes. Balloon expandable stents require mounting over a balloon, positioning, and inflation of the balloon to expand the stent radially outward. Self-expanding stents expand into place when unconstrained, without requiring assistance from a balloon. A self-expanding stent may be biased so as to expand upon release from the delivery catheter and/or include a shape-memory component which allows the stent to expand upon exposure to a predetermined condition. Some stents may be characterized as hybrid stents which have some characteristics of both self-expandable and balloon expandable stents.
Stents may be constructed from a variety of materials such as stainless steel, Elgiloy, nickel, titanium, nitinol, shape memory polymers, etc. Stents may also be formed in a variety of manners as well. For example a stent may be formed by etching or cutting the stent pattern from a tube or sheet of stent material; a sheet of stent material may be cut or etched according to a desired stent pattern whereupon the sheet may be rolled or otherwise formed into the desired substantially tubular, bifurcated or other shape of the stent; one or more wires or ribbons of stent material may be woven, braided or otherwise formed into a desired shape and pattern. The density of the braid in braided stents is measured in picks per inch. Stents may include components that are welded, bonded or otherwise engaged to one another.
A self-expanding stent is implanted in a blood vessel or other body lumen at the site of a stenosis or aneurysm by so-called “minimally invasive techniques” in which the stent is compressed radially inwards and is delivered by a catheter to the site where it is required through the patient's skin or by a “cut down” technique in which the blood vessel to be treated is exposed by minor surgical means. In such techniques, a microcatheter is threaded through the vascular system until its distal end reaches the implantation site. Then a compressed stent is introduced into the proximal end of the microcatheter and pushed through the microcatheter to its distal end using a pusher wire. When the stent is positioned at the correct location, it is pushed out of the microcatheter (i.e., unsheathed) and caused or allowed to expand to a predetermined diameter in the vessel.
Perceived problems with current stent delivery systems include inability to re-sheath stents that have been partially unsheathed, about 50-80%. Perceived problems also include lack of stability at the junction between the distal end of the pusher wire and the proximal end of the stent. Another perceived problem is that individual braid wires at both the proximal and distal ends of braided stents tend to flare out into the microcatheter wall as the stent is pushed through the microcatheter. Further, perceived problems include radial expansion of the stent during delivery as the stent is pushed through a microcatheter. The last two perceived problems increase frictional resistance as a stent is pushed through a microcatheter.